1. Field of the Invention
The present invention relates to a semiconductor light-receiving module having a semiconductor light-receiving element for converting an incoming optical signal from an incident light direction device into an electrical signal.
2. Description of the Related Art
Hitherto, a semiconductor light-receiving element composed of a semiconductor element for converting an optical signal into an electrical signal has been known.
FIG. 36 is a perspective view explaining a general configuration of a semiconductor light-receiving element composed of such semiconductor element for converting an incident light into current.
That is, as shown in FIG. 36, in this semiconductor light-receiving element, a lower cladding layer 5 made of n-InP is formed on a substrate 6 made of n+-InP.
Further on this lower cladding layer 5, a light absorbing layer 4 made of i-InGaAs, an upper cladding layer 3 made of p-InP, and a contact layer 2 made of p+-InGaAs are formed sequentially.
On the top face of the contact layer 2, a p-electrode 1 is provided.
On the bottom face of the substrate 2, an n-electrode 7 is provided.
Moreover, polyimide 8 for reducing the capacitance is formed in part of each side of the contact layer 2, upper cladding layer 3, light absorbing layer 4 and lower cladding layer 5, and in the lower part of the p-electrode 1.
The light absorbing layer 4 made of i-InGaAs, upper cladding layer 3 made of p-InP, and lower cladding layer 5 made of n-InP are combined to compose an optical waveguide for guiding the light entering inside from the light incident plane of the facet of this semiconductor light-receiving element as shown in FIG. 36.
In this optical waveguide, the refractive index of the light absorbing layer 4 is set higher than the refractive index of the upper cladding layer 3 or refractive index of the lower cladding layer 5.
That is, in the semiconductor light-receiving element of waveguide type, this light absorbing layer 4 functions as the core of playing the vital role of guiding the incident light.
In the semiconductor light-receiving element of this waveguide type, the width of mesa is about 4 μm, and the length is about 10 μm.
In the semiconductor light-receiving element having such configuration, while the incident light propagates through the optical waveguide composed of the light absorbing layer 4 made of i-InGaAs, upper cladding layer 3 made of p-InP, and lower cladding layer 5 made of n-InP, it is absorbed by the light absorbing layer 4, and is converted into an electrical signal.
At this time, the intensity I of the light is absorbed according to formula (1):I=I0exp(−αz)  (1)where I0 is power of the incident light on the light incident facet, α is an absorption coefficient, and z is a distance from the light incident facet.
FIG. 37 is a diagram showing the relation of the intensity I between the light propagating in the optical waveguide of the semiconductor light-receiving element and the distance z.
As understood from formula (1) and FIG. 37, when the light enters the optical waveguide, it is attenuated exponentially.
That is, the light is absorbed almost completely in a short distance from the light incident facet, and is converted into current.
In FIG. 37, the axis of abscissas represents the distance z from the light incident facet, and the axis of ordinates represents the intensity of the light propagating in the light absorbing layer 4, that is, the magnitude of the current caused as the incident light is absorbed by the light absorbing layer 4.
Joule heat occurring at each point in the light absorbing layer 4 is I2R where R is the load resistance, and therefore the fact that the light is absorbed almost completely in a short distance from the incident facet means that heat generation occurs suddenly in this short distance.
As a result, if the power of the incident light is large, the Joule heat generated in the short distance is extremely large, and this semiconductor light-receiving element may be broken.
Although not shown in the perspective view of FIG. 37 for the sake of simplicity of explanation, actually, to realize a multiple-mode optical waveguide, usually, a quaternary SCH (separate confinement heterostructure) layer of InGaAsP composition of about 1.3 μm in band gap wavelength is interposed above and beneath the light absorbing layer 4.
Accordingly, supposing the thickness of the light absorbing layer 4 to be 0.6 μm, and the total thickness of upper and lower SCH layers to be about 2 μm, the thickness of the crystal to be grown is as much as 3 to 4 μm in total, and it takes much time in crystal growth and the mesa etching process is complicated, thereby causing to limit the manufacturing yield of the semiconductor light-receiving element.
In order to eliminate such inconvenience, a semiconductor light-receiving element of loaded optical waveguide type as shown in FIGS. 38 and 39 is proposed.
FIG. 38 is a perspective view, and FIG. 39 is a cross sectional view.
In FIGS. 38 and 39, same reference numerals are given in the same parts as in the semiconductor light-receiving element shown in FIG. 36.
That is, as shown in FIGS. 38 and 39, in the semiconductor light-receiving element of this loaded optical waveguide type, a loaded optical waveguide layer 9 is formed on a substrate 6 made of n+-InP.
On this loaded optical waveguide layer 9, a light absorbing layer 4 made of i-InGaAs, an upper cladding layer 3 made of p-InP, and a contact layer 2 made of p+-InGaAs are formed sequentially.
On the upper side of the contact layer 2, a p-electrode 1 is provided.
On the bottom face of the substrate 6, an n-electrode 7 is provided.
Moreover, polyimide 8 for reducing the capacitance is formed in part of each side of the contact layer 2, upper cladding layer 3, light absorbing layer 4 and loaded optical waveguide layer 9, and in the lower part of the p-electrode 1.
The loaded optical waveguide layer 9 is made of a material of which refractive index is smaller than that of the light absorbing layer 4 and larger than that of the substrate 6, and, for example, n-InGaAsP having a band gap wavelength of 1.3 μm is used.
In such semiconductor light-receiving element of loaded optical waveguide type, as shown in FIGS. 38 and 39, light from outside enters the facet of the loaded optical waveguide layer 9.
Part of light propagating in this loaded optical waveguide layer 9 exudes to an adjacent light absorbing layer 4 and is coupled as shown in a cross sectional view in FIG. 39, and the photo responsivity of this light is proportional to the square of the absolute value of overlapping integration of an eigen-mode wave function φS of the loaded optical waveguide layer 9 and an eigen-mode wave function φC of the light absorbing layer 4, and is expressed in formula (2).Γ=|∫∫φS·(φC)*dxdy|2  (2)
FIGS. 40A and 40B are diagrams schematically showing the mode of evanescent coupling and absorbing of the light in the light absorbing layer 4 in the course of propagation through the loaded optical waveguide layer 9.
FIG. 40A shows a case where the thickness D of the loaded optical waveguide layer 9 is small (for example, D=0.7 μm).
FIG. 40B shows a case where the thickness D of the loaded optical waveguide layer 9 is large (for example, D=3 to 5 μm).
FIGS. 41A and 41B are diagrams showing the power I of the light propagating in the semiconductor light-receiving element as the function of the distance z in the light absorbing layer 4.
FIG. 41A shows a case where the thickness D of the loaded optical waveguide layer 9 is small (for example, D=0.7 μm).
FIG. 41B shows a case where the thickness D of the loaded optical waveguide layer 9 is large (for example, D=3/5 μm).
When the thickness D of the loaded optical waveguide layer 9 is small as shown in FIGS. 40A and 41A, the spot size of the light propagating in the loaded optical waveguide layer 9 is small, and the light propagating in the loaded optical waveguide layer 9 is strongly coupled evanescently to the light absorbing layer 4.
In other words, since the spot size of the light propagating in the loaded optical waveguide layer 9 is small, the value of the portion overlapping with the light absorbing layer 4 is greater in the wave function φS of the light propagating in the loaded optical waveguide layer 9, and hence the square of the absolute value of overlapping integration Γ expressed in formula (2) is also greater.
The spot size of the light entering the facet of the loaded optical waveguide layer 9 is set equal regardless of the thickness D of the loaded optical waveguide layer 9.
As a result, although the length La of the light absorbing layer 4 necessary for realizing the photo responsivity required as the semiconductor light-receiving element can be shortened, same as in the case of the semiconductor light-receiving element shown in FIG. 37, a great Joule heat is generated in a short distance z, and the element may be broken.
In order to avoid breakage of element by Joule heat, it is effective to weaken the degree of evanescent coupling of the light propagating in the loaded optical waveguide layer 9 to the light absorbing layer 4, that is, to increase the spot size of the light propagating in the loaded optical waveguide layer 9 by increasing the thickness D of the loaded optical waveguide layer 9 as shown in FIGS. 40B and 41B.
In other words, it is enough to decrease the value of the overlapping portion with the light absorbing layer 4 out of the wave function φS of the light propagating in the loaded optical waveguide layer 9.
In this case, however, since the square value Γ of the absolute value of the overlapping integration shown in formula (2) also becomes smaller, a longer length La of the light absorbing layer 4 is needed for realizing the photo responsivity required as the semiconductor light-receiving element.
Since the longer length La of the light absorbing layer 4 means the capacitance as the semiconductor light-receiving element becomes larger, the 3 dB bandwidth Δf (=1/(πRC)) becomes smaller and it is hard to apply this semiconductor light-receiving element in a high speed transmission system of, for example, 40 Gbps.
In this case, still more, since the thickness D of the loaded optical waveguide layer 9 is large, the total thickness of the semiconductor layers to be grown is large, and the load to crystal growth is extremely large.
Generally, when the crystal is grown thicker than 3 μm, strains are present in the semiconductor layer of crystal growth, and the film quality of the light absorbing layer 4 grown thereon deteriorates.
This deterioration of film quality is known to spoil the dark current characteristic as semiconductor light-receiving element indispensable for realizing a clear eye pattern in optical transmission.
Further, when the thickness D of the loaded optical waveguide layer 9 is increased and the spot size becomes larger, the increase of the upper cladding layer 3 must be also increased in order to reduce the light absorption loss due to the contact layer 2, and it becomes more difficult to achieve a favorable crystal growth.
Thus, in the semiconductor light-receiving element of loaded optical waveguide type shown in FIGS. 38 and 39, to realize high speed operation, if the spot size of the light propagating in the loaded optical waveguide layer 9 is decreased in order to shorten the length (element length) La of the light absorbing layer 4, the element is more likely to be broken by Joule heat. In addition, when the spot size of the light propagating in the loaded optical waveguide layer 9 is increased to avoid such thermal breakdown, the element length La becomes longer, and high speed operation is difficult, to the contrary.
In this semiconductor light-receiving element, moreover, it is hard to grow the light absorbing layer 4 of excellent crystallinity and small dark current on a thick grown crystal, and the characteristic as the semiconductor light-receiving element deteriorates.
To eliminate such inconvenience, it has been proposed to use a semiconductor light-receiving element of facet refractive type having a structure as shown in a cross sectional view in FIG. 42 (Jpn. Pat. Appln. KOKAI Publication No. 11-195807).
That is, as shown in FIG. 42, in this semiconductor light-receiving element of facet refractive type, a lower cladding layer 10 made of n+-InP, a light absorbing layer 4 made of i-InGaAs, an upper cladding layer 3 made of p-InP, and a contact layer 2 made of p+-InGaAs are sequentially formed on a substrate 11 made of a semi-insulating InP (SI-IP) material.
On the top face of the contact layer 2, a p-electrode 1 is provided.
On the lower cladding layer 10 made of n-InP, an n-electrode 7 is provided.
In this light-receiving element, as shown in FIG. 42, a light incident facet 12 formed of the facets of the substrate 11 and lower cladding layer 10 is inclined by wet etching.
The inclination angle of the light incident facet 12 to the top face of the lower cladding layer 10 (bottom face of the light absorbing layer 4) is about 54 degrees owing to the directivity of crystal at the time of etching.
The principle of operation of the semiconductor light-receiving element of facet refractive type having such configuration is explained below.
The incident light is refracted by the inclined incident facet 12, absorbed by the light absorbing layer 4, and converted into current.
Specifically, in the case of this semiconductor light-receiving element of facet refractive type, depending on the difference in refractive index between air (refractive index=1) and the lower cladding layer 10 made of n-InP (refractive index of InP is 3.17, but its refractive index is lowered to about 3.15 by n+ doping), the optical path of the light is changed obliquely upward, and passes obliquely above the light absorbing layer 4.
Thus, in the case of the semiconductor light-receiving element of facet refractive type, since the light passes the light absorbing layer 4 obliquely, the effective absorption length when passing the light absorbing layer 4 is longer, and the photo responsivity is enhanced.
Actually, however, the light passing obliquely above the light absorbing layer 4 further propagates obliquely above the upper cladding layer 3 and contact layer 2, and is reflected by the bottom face of the p-electrode 1, and the direction of optical path is changed obliquely downward.
The light of which optical path is changed obliquely downward passes again the contact layer 2 and upper cladding layer 3, and passes the light absorbing layer 4 obliquely downward.
Regrettably, however, in the contact layer 2 made of p+-InGaAs, since the absorbed light is not converted into current, it merely results in light absorption loss.
Further, at the time of reflection at the bottom face of the p-electrode 1, light scatter loss occurs due to roughness of the bottom face of the p-electrode 1.
Thus, the semiconductor light-receiving element of facet refractive type is likely to cause loss, and its photo responsivity is thereby low at 0.6 A/W or less.
In order to solve the problems of photo responsivity, a semiconductor light-receiving element having a structure as shown in a cross sectional view in FIG. 43 is proposed (Jpn. Pat. Appln. KOKAI Publication No. 2001-53328).
That is, as shown in FIG. 43, in this semiconductor light-receiving element, an InGaAsP layer 14, an n-InGaAsP layer 13, a light absorbing layer 4 made of i-InGaAs, an upper cladding layer 3 made of p-InP, and a contact layer 2 made of p+-InGaAs are sequentially formed on a substrate 11 made of a semi-insulating InP (SI-IP) material.
On the top face of the contact layer 2, a p-electrode 1 is provided.
On the n-InGaAsP layer 13, an n-electrode 7 is provided.
In this semiconductor light-receiving element, a light incident facet 12 formed of the facets of the InGaAsP layer 14 and n-InGaAsP layer 13 is inclined by wet etching.
The principle of operation of the semiconductor light-receiving element having such configuration is explained below while referring to FIG. 44.
Generally, the refractive index is decreased by n-doping, but for the sake of simplicity of explanation, herein, supposing the refractive index n13 of the n-InGaAsP layer 13 and the refractive index n14 of the InGaAsP layer 14 to be equal to each other (n13=n14=3.439), in FIG. 44, the n-InGaAsP layer 13 and InGaAsP layer 14 are represented by the InGaAsP layer 13.
In this semiconductor light-receiving element, all power of the light enters the InGaAsP layer 14 which is higher in the refractive index than the upper cladding layer 3.
As a result, the light is absorbed in the light absorbing layer 4 and propagates obliquely upward, and is totally reflected by the interface between the light absorbing layer 4 and the upper cladding 3.
As stated in claim 1 of Jpn. Pat. Appln. KOKAI Publication No. 2001-53328, “incident light entering from the lower semiconductor layer side passes the light absorbing layer obliquely in the film thickness direction, and is totally reflected by the interface of the first semiconductor layer at the light absorbing layer side, and passes the light absorbing layer again obliquely,” and the detailed description of the invention specifies “100% of light passes again the light absorbing layer and is absorbed,” and “the light passes the light-receiving layer two times and the effective light absorbing length is doubled.”
Further, in all embodiments illustrated in Jpn. Pat. Appln. KOKAI Publication No. 2001-53328, the light is once totally reflected, and propagates obliquely downward, and passes through the light absorbing layer.
In this semiconductor light-receiving element, as shown in FIG. 44, the light is absorbed in the light absorbing layer 4, and propagates obliquely upward, and 100% of the light is reflected at the interface between the light absorbing layer 4 and the upper cladding layer 3, and propagates the light absorbing layer 4 obliquely downward.
Therefore, as understood from FIG. 44, supposing the thickness of the light absorbing layer 4 to be T and the light passing angle to be φ, the effective absorption length Le for light is determined in formula (3).Le=2T/cos θ=2T/cos(π/2−φ)  (3)
That is, for example, if the thickness T of the light absorbing layer 4 is 0.4 μm, and the light passing angle φ is 25.8 degrees, the effective absorption length Le of light in this semiconductor light-receiving element is only about 4.6 times of the thickness T of the light absorbing layer 4 (because 1/cos θ=1/cos(π/2−25.8 degrees)=2.3), and it is insufficient for the absorption length.
Also as estimated from explanatory drawings of all embodiments and FIG. 43 described in Jpn. Pat. Appln. KOKAI Publication No. 2001-53328, in this semiconductor light-receiving element, 100% of light is reflected at the interface between the light absorbing layer 4 and the upper cladding layer 3, and propagates and passes obliquely downward in the light absorbing layer 4.
Therefore, in this semiconductor light-receiving element, the photo responsivity is not improved if the light absorbing layer 4 is extended over a specific length.
Further in this semiconductor light-receiving element, as understood from the described embodiments, all power of the light must be entered within the layer of high refractive index such as InGaAsP layer 14.
The light spot size (the radius of power of 1/e2) is 2 μm to 5 μm, that is, the diameter of power of 1/e2 is as wide as 4 μm to 10 μm.
As mentioned in relation to the semiconductor light-receiving element of loaded optical waveguide type in FIGS. 38 and 39, on a thick grown crystal, it is hard to grow a light absorbing layer of small dark current and favorable crystallinity.
On a GaAs substrate shown in the embodiment of Jpn. Pat. Appln. KOKAI Publication No. 2001-53328, fabrication of a structure in which InGaAsP or InGaAs for 1.55 μm bandwidth is crystal-grown is practically impossible from the viewpoint of lattice mismatching.
Therefore, the semiconductor light-receiving element shown in FIG. 43 is extremely difficult to manufacture, in reality.
As explained herein, in the semiconductor light-receiving element of waveguide type shown in FIG. 36, there is a problem in reliability of high optical input.
In the semiconductor light-receiving element of loaded optical waveguide type shown in FIGS. 38 and 39, there is a problem in reliability of high optical input, high speed response, and crystallinity of the light absorbing layer (in other words, dark current characteristic).
In the semiconductor light-receiving element of facet refractive type shown in FIGS. 42 and 43, there is a problem in photo responsivity and crystallinity of the light absorbing layer.